This article summarizes preliminary field evaluations of a new in situ geotechnical test, the torsional cylindrical impulse shear test. The impulse shear test provides, for soil deposits, detailed information on in situ nonlinear inelastic shearing deformation characteristics needed for dynamic geotechnical earthquake analysis procedures. The test addresses the issue of obtaining such information in a practical manner without disturbing the tested soils excessively. Herein, we present soil characteristics inferred from results of impulse shear tests conducted at four sites using a field prototype testing system. One of the sites is the I-10/La Cienega Blvd. undercrossing in Los Angeles, where a freeway structure collapsed during the 1994 Northridge earthquake, and another is the National Geotechnical Experimentation Site on Treasure Island in the San Francisco Bay. The four sites cover a broad range of soil conditions relevant to earthquake engineering. These include soft to medium-stiff clays and saturated loose sands. Comparisons between our results and published information suggest that the impulse shear test is a promising means for obtaining in situ nonlinear inelastic shearing deformation characteristics of soil deposits.